JPH08162659A - Solar cell - Google Patents

Abstract

PURPOSE: To optimize the novel structure of a solar cell having a high conversion efficiency and a large open circuit voltage or particularly the impurity density of a BSF layer and its thickness by setting the impurity density of the BSF layer to a specific value. CONSTITUTION: The solar cell comprises a first conductivity type window layer 44, and a first conductivity type emitter layer 43 formed under the layer 44. The cell further comprises a second conductivity type base layer 42 formed at the lower part of the layer 43, and a second conductivity type rear surface field layer (BSF layer) 41 formed at the lower part of the layer 42. Particularly, the impurity density of the layer 41 is 1 to 6×10<18> cm<-3> . Thus, the impurity density of the layer 41 is optimized to increase the long wavelength side optical sensitivity. As a result, the open circuit voltage and the short-circuiting current are increased together. The novel structure of the solar cell having high conversion efficiency and high electromotive force is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the structure of a solar cell, which is a semiconductor element for converting sunlight energy into electric energy, and particularly to a solar cell using a compound semiconductor mixed crystal devised to enhance the conversion efficiency. Regarding the structure of the battery.

[0002]

2. Description of the Related Art In a pn junction semiconductor solar cell comprising an n-type emitter layer 433 and a p-type base layer 422 as shown in FIG. 12 (a), the n-type emitter layer 433 is the light incident side and the p The photons absorbed in the type base layer 422 generate a pair of holes-electrons, of which electrons, which are minority carriers, move by diffusion and reach the depletion layer at the pn interface by the large electric field of the depletion layer. It flows into the n-type emitter layer 433 and becomes a current. However, Fig. 12 (b)
As shown in (4), some of the electrons generated in the p-type base layer 422 may enter the back surface electrode layer 8 by diffusion, and they can no longer return to the base layer 422, and some of them are holes that are majority carriers. It does not become a current by combining and disappearing. In order to prevent such back surface recombination loss, minority carriers (electrons) generated in the base layer 422 have been conventionally used.
Back surface field (Back Surface Field);
A layer called a "F" layer 411 is provided under the base layer 422. As shown in FIG. 12C, the BSF layer acts as a band barrier for the minority carriers generated in the base layer. As the BSF layer, 1) the same material as the base layer is used to increase the doping concentration to serve as a barrier against minority carriers, and 2) other semiconductor materials have a larger band gap than the base layer material, Also used is a barrier against minority carriers. With such a BSF layer, the conduction band of the base layer sharply rises at the interface with the BSF layer, and the electrons, which are minority carriers, are repelled by this barrier and cannot go further. In order to obtain a sufficient barrier against minority carriers by the method 1) above, it is necessary to make the doping concentration of the BSF layer extremely high. This is not a serious problem in the case of a single-layer solar cell, but a high-concentration doping layer is used as the upper cell BSF layer of a stacked solar cell in which solar cells are connected in series (tandem) to obtain a high output voltage. Is used, infrared light absorption due to a high concentration of majority carriers occurs in the layer. This infrared light is supposed to reach the cell in the lower layer of the tandem connection and be absorbed there to become a current, and such infrared light absorption in the BSF layer causes a large energy loss for the solar cell, which is not preferable. On the other hand, the above method 2) does not have a problem of infrared light absorption, but it is generally difficult to find a suitable material. Further, even if there is a suitable material, the base layer is grown on the BSF layer by hetero-heteroepitaxial growth, which has a drawback that crystal defects are likely to occur at the interface.

A solar cell (cell) as shown in FIG. 13 is known as a structure using the BSF layer according to the method 1). In FIG. 13, the surface electrode layer 7 is not shown.
In FIG. 13, a thickness of 20 nm is formed on the p ⁺ GaAs substrate,
P ⁺ In _0.49 Ga _0.51 P with an impurity density of 4 × 10 ¹⁷ cm ^-3
-BSF layer 41, 0.8 μm thick on this BSF layer 41
m, p-In _0.49 Ga with an impurity density of 1 × 10 ¹⁷ cm ⁻³
_0.51 P base layer 42, a thickness of 0.
N ⁺ In _0.49 G with 1 μm and impurity density of 2 × 10 ¹⁸ cm ⁻³
a _0.51 P emitter layer 43, with a thickness of 25n
An n-AlInP window layer 44 having m and an impurity density of 5 × 10 ¹⁷ cm ⁻³ is formed.

In the structure of FIG. 13, p-In _0.49 Ga _0.51 P
The forbidden band width Eg of the base layer 42 is 1.85 eV, and p-I
n _0.49 Ga _0.51 The forbidden band width Eg of the P-BSF layer 41 is 1.
It is 88 eV, which is a barrier against minority carriers. The method of 2) is shown in FIG.
_0.49 Ga _0.51 Forbidden band width Eg instead of P-BSF layer 41
There is also known an example using p-Al _0.06 Ga _0.45 In _0.49 P of 1.95 eV.

[0005]

However, in the conventional method,
A cell using the Al _0.06 Ga _0.45 In _0.49 P-BSF layer has a large conversion band and an open-circuit voltage (open-circuit voltage) despite the large forbidden band width of the BSF layer.
It had the drawback that the ltage) Voc was extremely low.
It is considered that this is because oxygen is mainly taken into the BSF layer due to the gettering action of Al and the crystallinity is deteriorated. In addition, p-I having an impurity density of 4 × 10 ¹⁷ cm ⁻³
n _0.49 Ga _0.51 Al _0.06 Ga _0.45 also in the case of P-BSF layer
Although it is slightly better than the case of In _0.49 P, it has the drawbacks of lower Voc and lower conversion efficiency than theoretically predicted values.

The present invention has been made in view of the above points, and has a novel structure of a solar cell having a high conversion efficiency and a large open circuit voltage Voc, in particular, an impurity density of a BSF layer and a solar having an optimized thickness. The purpose is to provide a battery.

[0007]

In order to solve the above problems, the first feature of the present invention is that, as shown in FIG. 1, a first conductivity type emitter layer 43 and a second conductivity type base layer 42 are provided. , A second-conductivity-type BSF layer 41 at least, and the BSF layer 41 has an impurity density of 1 to 6 × 10 ¹⁸ cm ⁻³ .

Preferably, in the first feature, the emitter layer 43, the base layer 42 and the BSF layer 41 are made of In _1-x Ga _x P.
And more preferably the BSF layer has an impurity density of 2
It is about 3 × 10 ¹⁸ cm ⁻³ .

A second feature of the present invention is that, as shown in FIG. 6, a first conductivity type first emitter layer 43, a second conductivity type first base layer 42, and a second conductivity type first emitter layer 43. 1 BSF layer 41
At least, a second cell layer 22 of the first conductivity type, a second base layer 22 of the second conductivity type, and a second BSF layer 21 of the second conductivity type. The bottom cell 2 included therein is formed between the top cell 4 and the bottom cell 2, and the top cell 4 and the bottom cell 2 are provided.
A solar cell having a tandem structure including at least a tunnel junction 3 for connecting a first BS and
The impurity density of the F layer is 1 to 6 × 10 ¹⁸ cm ⁻³ .

Preferably in the second feature, the first B
That is, the impurity density of the SF layer 41 is 2-3 × 10 ¹⁸ cm ⁻³ .

More preferably, in the second feature, the first emitter layer 43, the base layer 42, the BSF layer 41, and the second BSF layer 21 are made of In _1-x Ga _x P, and the second emitter layer 23. The base layer 22 is made of GaAs.

[0012]

The solar cell having the first characteristic of the present invention has the BSF layer 4
By optimizing the impurity density of No. 1, the photosensitivity on the long wavelength side is increased as shown in FIG. 5, and as a result, FIGS.
Open circuit voltage Voc, short circuit current (short-
The circuit current (Isc) increases together.

The solar cell of the second feature of the present invention is obtained by applying the solar cell of the first feature to a tandem type solar cell. By optimizing the impurity density of the first BSF layer 41, 8, short-circuit photocurrent density Js as shown in FIG.
Both c and the open circuit voltage Voc increase, and the conversion efficiency improves.

The BSF layer 41 of the first feature,
If the impurity density of the first BSF layer 41 in the second feature is made too high, the crystallinity of the BSF layer 41 is lowered, the interface state with the base layer 42 is deteriorated, and the carrier life (lifetime) is reduced. , The conversion efficiency will decrease, so BS
It is not preferable that the impurity density of the F layer 41 exceeds 6 × 10 ¹⁸ cm ⁻³ .

[0015]

Embodiments of the present invention will be described below with reference to the drawings. The same reference numerals are used for the portions overlapping in FIGS. FIG. 1 shows GaAs according to the first embodiment of the present invention.
A part of the cross-sectional structure of an In _0.5 Ga _0.5 P single cell lattice-matched to is shown. In _1-x Ga _x P has excellent radiation resistance and is expected to be applied to space applications, but it is also promising as a top cell of a tandem cell described later. In
_{This is because 1-x} Ga _x P has a high band gap and a high absorption coefficient. In FIG. 1, (100) 5 ° off [011]
Direction impurity concentration 1 × 10 ¹⁹ cm ⁻³ Zn-doped p ⁺ G
On an aAs substrate 11, a thickness of 0.5 μm and an impurity density of 5 ×
Through the 10 ¹⁸ cm ⁻³ p ⁺ GaAs buffer layer 12,
P of 0.3 μm thickness and impurity density of 1 to 6 × 10 ¹⁸ cm ^{−3
A +} In _0.5 Ga _0.5 P back surface field layer (BSF layer) 41 is formed. The impurity density of the BSF layer 41 is 2-3 × 1.
0 ¹⁸ cm ^-3 is preferable. Furthermore, a thickness of 0.7 to 1.5 μm and an impurity density of 2 to 3 × 10 ¹⁷ cm ⁻³ are formed on the BSF layer 41.
PIn _0.5 Ga _0.5 P base layer 42; thickness 50 nm,
N ⁺ In _0.5 Ga _0.5 P with an impurity density of 3 × 10 ¹⁸ cm ^-3
Emitter layer 43; thickness 30 nm, impurity density 2 × 10 ¹⁸
A cm ⁻³ n ⁺ In _0.5 Ga _0.5 P window layer 44 is formed in this order. The n ⁺ In _0.5 Ga _0.5 P window layer 44 has a thickness of 0.3 μm for ohmic contact on a part of the upper portion thereof.
N ⁺ GaAs layer 51 and Au—Ge / Ni / Au layer 71 having an impurity density of 5 × 10 ¹⁸ cm ⁻³ and A having a thickness of 1 μm
Upper metal electrode layer (surface electrode layer) including the u-plated layer 72
7 are formed. FIG. 2 is a plan view of the In _0.5 Ga _0.5 P cell according to the first embodiment of the present invention, in which the occupied area of the upper metal electrode layer 7 is 2% of the total surface area, and the others are light receiving surfaces. An antireflection film 6 made of a ZnS film 61 having a thickness of 55 nm and a MgF ₂ film 62 having a thickness of 95 nm is formed on the surface of FIG. 1 where the upper metal electrode layer 7 is not formed. A lower metal electrode layer (rear surface electrode layer) 8 made of an Au plating layer having a thickness of 1 μm is formed on the rear surface of the p ⁺ GaAs substrate 11. The p ⁺ GaAs buffer layer 12 and the p ⁺ In _0.5 Ga _0.5 P-BSF layer 4
1. pIn _0.5 Ga _0.5 P base layer has a dopant of Zn
Is a typical example of n ⁺ In _0.5 Ga _0.5 P emitter layer 43,
Si is generally used as the dopant for the n ⁺ AlInP window layer 44 and the n ⁺ GaAs layer 51, but other dopants may be used.

FIG. 3 shows a base layer 42 having a thickness of 1.5 μm.
The IV characteristics of the In _0.5 Ga _0.5 P cell without the antireflection film 6 are shown with the impurity density of the BSF layer 41 as a parameter. Compared with the case where the impurity density of BSF of the prior art is 2 × 10 ¹⁷ cm ⁻³ , the electromotive current is larger in the case of the present invention, and particularly the impurity density of the BSF layer is 2 × 1.
In the case of 0 ¹⁸ cm ^-3, the electromotive current is large and the maximum efficiency is 18.4.
8% was obtained. FIG. 4 shows that the base layer 42 has a thickness of 0.7 μm.
m, and the BSF layer 41 has a thickness of 0.3 μm, the open circuit voltage (open-ci) in a state in which the antireflection film 6 is not attached.
rcuit voltage) Voc and short-circuit current Is
The dependence of c on the impurity density of the BSF layer 41 is shown. By increasing the impurity density of the BSF layer 41, Voc, Isc
It can be seen that both improve. FIG. 5 shows the relationship between the external quantum efficiency and the wavelength of incident light, that is, the spectral sensitivity characteristic, and shows the impurity density of the BSF layer 41 as a parameter. It was found that increasing the impurity density of the BSF layer 41 improves the long-wavelength photosensitivity, which is shown in FIG.
It is considered that the increase in sc is brought about.

In order to obtain a higher conversion efficiency or a higher electromotive voltage, a so-called tandem cell structure may be formed by connecting a plurality of single cells in series as shown in FIG. FIG. 6 shows InGaP according to the second embodiment of the present invention.
1 shows the structure of a / GaAs tandem cell. In FIG. 6, a GaAs cell 2 is used as the lower cell (bottom cell), and an In _0.5 Ga _0.5 P cell 4 similar to that shown in the first embodiment of the present invention is used as the upper cell (top cell). Has been. Further, in order to electrically connect the lower cell and the upper cell in series, GaAs is provided between them.
The tunnel junction layer 3 is provided. Tunnel junction layer 3
Has a peak current of about 50 mA / cm ² . More specifically, the GaAs bottom cell 2 is Zn-doped p ⁺ GaAs.
A p having a thickness of 0.3 μm and an impurity density of 7.0 × 10 ¹⁸ cm ⁻³ formed on the substrate 11 (p <1 × 10 ¹⁹ cm ⁻³ ).
⁺ Formed on the GaAs buffer layer 12. The GaAs cell 2 has a thickness of 0.1 μm formed on the p ⁺ GaAs buffer layer 12 and an impurity density of 3.0 × 10.
¹⁸ cm ⁻³ p ⁺ InGaP BSF layer 21, thickness of 3 μm provided on the BSF layer 21, impurity concentration of 2.0 × 10 ¹⁷ cm
^-3 pGaAs base layer 22, n ⁺ with a thickness of 0.1 μm provided thereon and an impurity density of 2.0 × 10 ¹⁸ cm ⁻³
The thickness of the GaAs emitter layer 23, and the upper part thereof is 0.1.
μm, n ⁺ InGa with an impurity density of 2.0 × 10 ¹⁸ cm ⁻³
And a P window layer 24. A pn junction is formed between the emitter layer 23 and the base layer 22. GaAs
The tunnel junction layer 3 is a lower cell (GaAs bottom cell) 2
N ⁺ GaAs layer 31 having a thickness of 15 nm and an impurity density of 5 × 10 ¹⁸ cm ⁻³ or more formed on the uppermost n ⁺ InGaP window layer 24, and a thickness of 15 nm and an impurity density of 1.
It is composed of 0 × 10 ¹⁹ p ⁺ GaAs layer 32. And, on the top of this, with a thickness of 0.1 to 0.5 μm and an impurity density of 1 to 6 × 10 ¹⁸ cm ⁻³ of p ⁺ In _0.5 Ga _0.5.
B BSF layer 41 of P; thickness 0.7 to 1.5 μm, impurity density 1.5 × 10 ¹⁷ cm ⁻³ pIn _0.5 Ga _0.5 P base layer 42; thickness 50 nm, impurity density 3.0 × 10 ¹⁸ cm
^-3 n ⁺ In _{0. 5} Ga _0.5 P emitter layer 43; and the thickness of 30 nm, the impurity density of ^{2 × 10 18 cm -3 n +} AlI
In _0.5 Ga _0.5 P with nP window layer 44 deposited in this order
The top cell 4 is formed. An n ⁺ GaAs layer 51 having a thickness of 0.3 μm for ohmic contact is formed on a part of an upper portion of the In _0.5 Ga _0.5 P top cell 4, and an Au—Ge / Ni / Au layer 71 and an upper portion thereof are formed on the n ⁺ GaAs layer 51. An upper metal electrode layer (surface electrode layer) 7 made of the Au layer 72 is formed. On the back surface of the p ⁺ GaAs substrate 11, an Au layer is formed as a lower metal electrode layer (back surface electrode layer) 8. In _0.5 Ga _0.5 P n ⁺ AlIn of top cell 4
An antireflection film 6 composed of a ZnS layer 61 and a MgF ₂ layer 62 is formed on a region of the surface of the P window layer 44 other than a portion where the n ⁺ GaAs layer 51 and the upper metal electrode layer 7 are formed thereon. ing. In addition, as in the first embodiment of the present invention, p ⁺ In of the In _0.5 Ga _0.5 P top cell 4 is formed.
The impurity density of the _0.5 Ga _0.5 P-BSF layer 41 is 2 to 3 ×.
More preferably, it is 10 ¹⁸ cm ^-3 . p ⁺ In _0.5 G
If the thickness of the a _0.5 P-BSF layer 41 is 0.5 μm or more, light is absorbed and the efficiency is lowered, which is not desirable. Figure 6
Is a p ⁺ GaAs buffer layer 12, p
⁺ InGaP-BSF layer 21, pGaAs base layer 2
2, p ⁺ GaAs layer 32, p ⁺ In _0.5 Ga _0.5 P-B
Zn for the SF layer 41 and the pInGaP base layer 42
Is preferably used, and the n ⁺ GaAs emitter layer 23,
n ⁺ In _0.5 Ga _0.5 P window layer 24, n ⁺ GaAs layer 3
1, n ⁺ In _0.5 G _0.5 P emitter layer 43, n ⁺ AlI
It is desirable to use Si for the nP window layer 44 and the n ⁺ GaAs layer 51, but similar effects can be obtained with other dopants.

FIG. 7 shows In _0.5 Ga in a tandem configuration.
The thickness of the BSF layer 41 of the _0.5 P top cell 4 is 5 × 10 ¹⁷ cm ⁻³ , 7 × 10 ¹⁷ cm ⁻³ , 2 × 10 ¹⁸ c, and the impurity density is 5 × 10 ¹⁷ cm ⁻³ .
The spectral sensitivity characteristics when changed to m ^-3 are shown. Figure 8
The short-circuit photocurrent density Jsc of the tandem cell is plotted against the impurity density of the BSF layer 41.
The impurity density of 2 × 10 ¹⁸ cm ⁻³
Then, it can be seen that the Jsc of the tandem cell increases. FIG. 9 shows the Voc BSF layer 41 in the tandem cell.
When the impurity density of the BSF layer 41 is 2 × 10 ¹⁸ cm ⁻³ , Voc becomes maximum and is 2.3.
It was 8V. In FIG. 9, the ● mark indicates that the growth temperature of the GaAs bottom cell 2 is 650 ° C., and the □ mark indicates that the growth temperature is 700 ° C.
Is the case. The growth temperature of the GaAs bottom cell 2 is 70
By setting the temperature to 0 ° C., the Voc of the tandem cell is 2.4.
It turns out that it becomes 1 or more. FIG. 10 shows optical IV characteristics and output characteristics of the tandem cell in the second embodiment of the present invention. According to the second embodiment of the present invention, a conversion efficiency of 27.3% can be obtained.

The single cell and tandem cell solar cells shown in the first and second embodiments of the present invention can be manufactured by the manufacturing method shown in FIG. Since the first and second embodiments are almost the same manufacturing method, they will be described in the second embodiment.

(A) First, as shown in FIG. 11 (a), metal organic chemical vapor deposition (MOCVD), CBE (Chemical)
Beam Epitaxy) method, MBE (Molecular Beam Epitax)
y) method, MLE (Molecular Layer Epitaxy) method, etc., is used to form the p ⁺ GaAs layer 1 on the p ⁺ GaAs substrate 11.
2, GaAs bottom cell 2, GaAs tunnel junction 3,
In _0.5 Ga _0.5 P top cell 4, n ⁺ GaAs layer 51
Are continuously epitaxially grown. More specifically, as shown in FIG. 6, the GaAs bottom cell 2 is p ⁺ InGaP −.
BSF layer 21, pGaAs base layer 22, n ⁺ GaAs
It is a multilayer epitaxial growth layer of the emitter layer 23 and the n ⁺ In _0.5 Ga _0.5 P layer 24, and the In _0.5 Ga _0.5 P top cell 4 is a p ⁺ In _0.5 Ga _0.5 P-BSF layer 41,
pIn _0.5 Ga _0.5 P base layer 42, n ⁺ In _0.5 Ga
It is a multilayer epitaxial growth layer in which a _0.5 P emitter layer 43 and an n ⁺ AlInP window layer 44 are laminated in this order.
The aAs tunnel junction is composed of n ⁺ GaAs layer 31, p ⁺ GaA
It is a continuous epitaxial growth layer composed of the s layer 32. M
OCVD can be performed by atmospheric pressure MOCVD or reduced pressure MOCVD, but it is preferable that the reduced pressure MOCVD method is maintained at, for example, 6.7 to 10 kPa, and more preferable that the vertical type reduced pressure MOCVD method is used. Group III source gases include triethylgallium (TEG), trimethylindium (TMI), trimethylaluminum (TMA),
Trimethylamine alane (TMAAl) and other group V source gases include phosphine (PH ₃ ) and arsine (As).
H ₃ ) or the like is used. Or tertiary butyl
Phosphine ((C ₄ H ₉ ) PH ₂ ; TBP), tertiary butyl arsine ((C ₄ H ₉ ) AsH ₂ ; T
BA) or the like may be used. As the n-type dopant gas, monosilane (SiH ₄ ) or disilane (Si
₂ H ₆ ), diethyl selenium (DESe), diethyl tellurium (DETe) or the like may be used, but monosilane is preferable. Diethyl zinc (DEZn) or trimethylgallium (TM) is used as the p-type dopant gas.
G) may be used. The raw material gas and the dopant gas are 6.7 kP by using a mass flow controller or the like.
It is introduced into a reaction tube controlled to a reduced pressure of a to 10 kPa. The ratio of the group V source gas to the group III source gas, the so-called V / III ratio, may be about 120 to 170, for example. The substrate temperature during growth is, for example, 650 ° C. to 700 ° C.
In order to obtain a high Voc value as shown in FIG. 12, the GaAs bottom cell 2 is grown at 700 ° C.
Is preferred.

(B) Next, the wafer having the multilayer structure thus continuously epitaxially grown is taken out from the reaction tube, a photoresist for lift-off is applied, a predetermined pattern is formed by photolithography, and Au-- Vacuum deposition of Ge / Ni / Au. For example, 10
0 nm Au-Ge (12 wt%), 20 nm Ni,
A 70 nm Au film is formed by the EB vapor deposition method. After that, if the photoresist is removed, the photoresist shown in FIG.
An upper metal electrode layer 71 having a comb-like stripe shape as shown in FIG. Although a similar pattern can be obtained by etching with an etchant such as a KI / I solution by ordinary photolithography without using the lift-off method, the lift-off method is simpler. Then, the electrode is sintered at 360 to 450 ° C. in an H ₂ atmosphere or an inert gas atmosphere such as N ₂ . Sintering at 360 ° C. for about 2 seconds is preferable.

(C) Next, the surface of the epitaxial growth layer is covered with photoresist or the like, and the back surface of the p ⁺ GaAs substrate 11 is etched by about 6 μm with a bromine (Br ₂ ) based solution or the like, and then a back electrode of about 1 μm is formed on the surface. Layer 8 is Au plated. Then, Au-G on the surface of the epitaxial growth layer
A window is opened only in the e / Ni / Au film 71 using photolithography, the other part is covered with a photoresist, and an Au plating film 72 of about 1 μm is plated to obtain the shape of FIG. 14C. If the electroplating method is used, this photolithography can be omitted. The step of forming the upper metal electrode layer 71 shown in FIG. 11B may be performed after the step of forming the back electrode layer 8.

(D) Next, for example, 10 m shown in FIG.
The main area and the back surface of the element consisting of the light receiving surface of m × 20 mm and the upper electrode layer are covered with photoresist, and FIG.
As shown in (d), a predetermined portion of the epitaxial growth layer is mesa-etched with a width of about 30 μm to 50 μm to form a mesa 9. Eventually, a large number of 10 mm × 20 mm islands will be surrounded by the mesa region. HCl for mesa etching
System etchant and ammonia / hydrogen peroxide (H ₂
O ₂ ) may be used. A sulfuric acid-based etchant or a tartaric acid-based etchant may be used, but a bromine-based etchant is preferably used for etching by about 1 μm.

Subsequently, using the pattern of the upper metal electrode layer 7 as a mask, only the n ⁺ GaAs layer 51 under the upper electrode layer 7 is left, and the other portions of the n ⁺ GaAs layer 51 are removed. This etching is performed with ammonia / hydrogen peroxide, and GaAs may be selectively etched for about 30 seconds.

(E) Next, using the lift-off method again, Z
The antireflection film 6 including the nS film 61 and the MgF ₂ film 62 is formed. That is, the ZnS film is deposited to a thickness of about 55 to 65 nm by sputtering, and then the MgF ₂ film is deposited to 95 nm by the EB evaporation method.
-120 nm is formed.

(F) Next, although illustration is omitted, a width of 30 μ
A scribe line is drawn in the center of the mesa line of m to 50 μm, and a cell of 10 × 20 mm is cut out by cleavage to complete. If the p ⁺ GaAs substrate 11 is a 2-inch wafer, 6 cells of 10 mm × 20 mm can be cut out. The cell size is an example, and may be set as needed.
For example, if InGaP is grown on a Si substrate, a large-area solar cell having a diameter of 4 inches to 8 inches can be obtained at a low price.

[0027]

According to the present invention, the spectral sensitivity characteristics on the long wavelength side are improved, and the single cell has a 18.
Conversion efficiency of 5% or more and tandem cell of 27.3% or more is obtained, which is extremely high efficiency.

Further, according to the present invention, the spectral sensitivity on the long wavelength side is improved, and Voc is also greatly improved. Voc of the In _0.5 Ga _0.5 P single cell is 1.39, In _0.5 Ga _0.5 P / Ga
The Voc of As tandem cell is as high as 2.418V. According to the present invention, fill factor (fill f
(actor; FF) is also improved.

[Brief description of drawings]

FIG. 1 shows In _0.5 Ga _{0.5 according} to the first embodiment of the present invention.
It is sectional drawing of P single cell.

FIG. 2 shows In _0.5 Ga _{0.5 according} to the first embodiment of the present invention.
It is a top view of a P single cell.

FIG. 3 shows In _0.5 Ga _{0.5 according} to the first embodiment of the present invention.
FIG. 7 is an optical IV characteristic diagram of a P single cell.

FIG. 4 shows In _0.5 Ga _{0.5 according} to the first embodiment of the present invention.
It is a figure which shows the thickness dependency of the BSF layer of Isc of a P single cell, and Voc.

FIG. 5 shows In _0.5 Ga _{0.5 according} to the first embodiment of the present invention.
It is a spectral sensitivity characteristic view of P single cell.

FIG. 6 shows In _0.5 Ga _{0.5 according} to the second embodiment of the present invention.
It is a sectional view of a P / GaAs tandem cell.

FIG. 7 is a diagram showing the dependency of spectral sensitivity characteristics on the impurity density of a BSF layer.

FIG. 8 is a diagram showing the dependency of Jsc on the impurity density of the BSF layer.

FIG. 9 is a relationship diagram between the Voc of the tandem cell and the impurity density of the BSF layer.

FIG. 10 is an output characteristic diagram of the tandem cell according to the second embodiment of the present invention.

FIG. 11 is a diagram illustrating a method of manufacturing a tandem cell according to the second embodiment of the present invention.

FIG. 12 is a schematic diagram illustrating a solar cell according to a conventional technique.

FIG. 13 is a diagram illustrating a conventional solar cell.

[Explanation of symbols]

2 GaAs bottom cell (lower cell) 3 GaAs tunnel junction 4 In _0.5 Ga _0.5 P top cell (upper cell) 6 antireflection film 7 upper metal electrode layer (front electrode layer) 8 lower metal electrode layer (back electrode layer) 9 mesa 11 p ⁺ GaAs substrate 12 p ⁺ GaAs buffer layer 21 p ⁺ In _0.5 Ga _0.5 P BSF layer 22 p GaAs base layer 23 n ⁺ GaAs emitter layer 24 n ⁺ InGaP window layer 31, 51 n ⁺ GaAs layer 32 p ⁺ GaAs layer 41 p ⁺ In _0.5 Ga _0.5 P-BSF layer 42 pIn _0.5 Ga _0.5 P base layer 43 n ⁺ InGaP emitter layer 44 n ⁺ AlInP window layer 61 ZnS film 62 MgF ₂ film 71 Au-Ge / Ni / Au film 72 Au plated film 422 Base layer 433 Emitter layer

Claims (6)

[Claims] 1. A first conductivity type emitter layer, a second conductivity type base layer formed under the emitter layer, and a second conductivity type back surface electric field layer formed under the base layer ( And a compound semiconductor solar cell, wherein the BSF layer has an impurity density of 1 to 6 × 10 ¹⁸ cm ⁻³ . 2. The impurity density of the BSF layer is 2 to 3 × 1.
The solar cell according to claim 1, which is 0 ¹⁸ cm ^-3 . 3. The solar cell according to claim 1, wherein the emitter layer, the base layer and the BSF layer are In _1-x Ga _x P. 4. A first conductivity type first emitter layer and a second emitter layer.
Conductive type first base layer and second conductive type first BSF
Bottom cell having at least a top cell having at least a layer, a second emitter layer having a first conductivity type, a second base layer having a second conductivity type, and a second BSF layer having a second conductivity type And a tandem structure solar cell comprising at least a tunnel junction formed between the top cell and the bottom cell and connecting the top cell and the bottom cell to each other, wherein the impurity density of the first BSF layer is Is 1 to 6 × 10 ¹⁸ c
A solar cell characterized by being m ^-3 . 5. The impurity density of the first BSF layer is 2 to
The solar cell according to claim 4, which has a size of 3 × 10 ¹⁸ cm ⁻³ . 6. The first emitter layer, base layer, BS
The solar cell according to claim 4 or 5, wherein the F layer and the second BSF layer are In _1-x Ga _x P, and the second emitter layer and the base layer are GaAs.